A difference in electric load between day and night or among seasons is a large problem for an electricity supply system, and recently the difference in electric load increases more and more. On the other hand, in order to improve industrial competitiveness and the like, a demand for reducing electricity cost increases, and a load leveling applied to the electricity supply becomes a considerably large issue. A highly efficient with large-capacity electricity supply system is being developed and put into practical use as one of countermeasures against the issue.
In order to solve the problem with the decrease of the electricity cost, for example, Patent Document 1 discloses a technique relating to a hybrid battery system in which a sodium-sulfur battery having high energy capacity and a high-output type battery having a high output/energy ratio relative to the sodium-sulfur battery are connected in parallel with respect to an electricity system.    Patent Document 1: Japanese Patent Publication Laid-Open No. 2004-215456